Endothermic gas is used, e.g., as a protective atmosphere for the heat treatment of various metals and is also used as a carrier gas for carburizing. Most commonly, endothermic gas is prepared in an endothermic gas generator by reacting hydrocarbon gas and air in a reaction retort containing a catalyst at elevated temperature.
The composition of the endothermic gas is determined, inter alia, by the ratio of input air to hydrocarbon gas supplied to the retorts. Automatic systems for controlling the composition of an endothermic gas product are known. For example, a known fixed air/gas ratio control system has been used to set the ratio of hydrocarbon gas to air. These systems typically use orifices to control the gas and air input lines. These systems also make use of manual or automatic flap or “butterfly” valves, called trim valves, to make minor adjustments to that ratio in an attempt to keep the product endothermic gas at the desired composition.
In these prior mixing systems, the amount of trim control available is defined by the physical characteristics of the trim valves. A trim valve presents a fixed maximum orifice size when fully opened. Air velocity flow through a given system can vary significantly depending upon the demand for the air-gas mixture by the endothermic gas generator. Due to their physical characteristics, a trim valve may provide desired sensitivity and trim control at high flow rates through the retorts, but not at lower flow rates. For example, when fully opened, a trim valve may accommodate a flow of 600 cubic feet per hour (cfh). As FIG. 9 shows, at a generator demand of 6000 cfh, this represents a desirable maximum sensitivity, or trim value, of 10% (600 cfh/6000 cfh). However, as FIG. 9 demonstrates, at a lesser generator demand of 3000 cfh, the same trim valve offers a less desirable sensitivity or trim value of 20% (600 cfh/3000 cfh). At a still lesser generator demand of 1000 cfh, the same trim valve offers a much less desirable sensitivity of 60% (600 cfh/1000 cfh). Thus, while conventional mixing technology using trim valves may provide the requisite sensitivity at high demand flow rates, they do not provide the same sensitivity desired at lower demand flow rates. Turndown of generator output is desired to eliminate producing excess endothermic gas. Excess endothermic gas is typically wasted, thus increasing the cost of production. With conventional mixing technology, the desired degree of sensitivity and trim control at low demand flow rates cannot be achieved without separate control loops entailing additional control valves and complex logic circuitry.